Hate it or not, Apple has to make it relatively commonplace if they want macOS apps to bother developing for it instead of ignoring it. Their short-term goal is 100% of MBP users, and surely, eventually 100% of all macOS laptop users.
Keeping it optional indefinitely, then, defeats this goal.
Then is it going to come to desktop machines too, where people often use battery-powered Bluetooth keyboards, or peripherals not made by Apple? What about the tiny 12 inch Macbook?
I don't see how this thing is practical across the entire Mac line.
The 12-inch Macbook has an F-key row which is what gets replaced with the touch bar.
I only estimated that they were going for 100% laptop coverage. But I can imagine a future where Apple keyboards have a touch bar option. Though not as important because Macbook touch bar penetration can drive developers to integrate with it, alone.
At which point it's not much different than gestures when it comes to answering your questions. What happens when you use a Logitech mouse on your macOS desktop instead of a Magic Trackpad?
Note that touch bar integration cannot have unique features, so it's never required. The challenge is to get developers to care about it which is the prerequisite for users to care about it.
I don't see how you can add the Touchbar to desktops without creating a wired keyboard or adding an expensive big battery to it and jacking the price to $200.
Actually, I do: They'll raise the price of the cheapest iMac configuration and include the keyboard in the box. After seeing the starting Mac Mini prices, it makes total sense.
Keeping it optional indefinitely, then, defeats this goal.